The invention relates to a process and apparatus for measuring the alcohol content of the damping fluid for alcohol damping systems such as those used in offset printing presses, in order to keep the alcohol content or concentration at a constant set value. This is done by injecting alcohol into the damping fluid, in addition to the conventional addition of alcohol as determined by experimental values or an addition of alcohol according to parameters set through experimental values. This additional injection of alcohol results in a sensitive adjustment of the set value, while the constant or parameter dependent addition of alcohol results in a rough adjustment.
The measurement of alcohol content in damping agents of prior art alcohol damping systems in offset printing presses has involved measuring the density of the damping agent through hydrometering or aerometering for example. This type of measurement is not very exact because it does not take into account that the specific weight or density of the damping agent does not depend only on the alcohol content (usually isopropyl alcohol, i.e.: using 2-propyls), but also to a large extent on the amount of additives utilized for the purpose of maintaining a specific pH value, typically in the range of 4 to 6.5 for example. Generally, such additives are supplied to the damping agent for the protection of the platen and surface substances including antimicrobial ingredients, complexing agents to compensate for high water hardness, anticorrosive inhibiting substances, and others.
Such additive substances influenece the specific weight of the damping medium in such a way that the alcohol content determined by hydrometer may be 50% to 100% higher than the actual alcohol content determined by chemical analysis. Excessive alcohol content is not only a waste of rather costly alcohol, it also increases immission, etching of the color and a generally less than optimal print work. Consequently it is desirable to work with the lowest possible alcohol content, preferably in the range of 6%, for example. At such low levels of alcohol content, it is absolutely necessary to stay as close as possible to the set value, since the impact of a 1% alcohol content variation is far greater at lower set values than if the set value were higher, for example 15%.